It's another late night, and I'm confused. I've tried a bunch of things, and I'm willing to learn something here, there's definitely no big money waiting in this 525! Aaahh, what comes with learning is much sweeter!
Attached is a schematic. The 525 was found with 20VDC on spkr terms. That means 20V across the 2200u coupling caps. One was puffed out, so both were replaced. 20VDC (confirmed with scope) still passes across the cap.
I disconnected the 2200u's from the circuit. 20V still on terms. The only other schematic connection is a NFB circuit, pins 11 + 12, which couple the Pwr amp signal to base xs bias via 2.2uf caps.
Next, with the 2200u still disconnected, I removed these 2.2u caps. 20V went away. Live circuit test, I reinserted a 2.2u and found 20V show up on the scope, where the terminals would be. All caps described during my tests are new. Am I missing something? Thanks.
Attached is a schematic. The 525 was found with 20VDC on spkr terms. That means 20V across the 2200u coupling caps. One was puffed out, so both were replaced. 20VDC (confirmed with scope) still passes across the cap.
I disconnected the 2200u's from the circuit. 20V still on terms. The only other schematic connection is a NFB circuit, pins 11 + 12, which couple the Pwr amp signal to base xs bias via 2.2uf caps.
Next, with the 2200u still disconnected, I removed these 2.2u caps. 20V went away. Live circuit test, I reinserted a 2.2u and found 20V show up on the scope, where the terminals would be. All caps described during my tests are new. Am I missing something? Thanks.
Attachments
Do you load the output with a resistor or a loudspeaker or only with the scope? The time constant of 2200 uF with a 1 megohm or 10 megohm oscilloscope probe is 2200 or 22000 seconds, respectively. That is, without extra load you have to wait for hours for the capacitors to get charged - and if their leakage is as bad as electrolytic capacitor manufacturers specify, they'll never get fully charged at all.
Thank You MarcelvdG. I will think about RC time constants more frequently now.
This is my first capacitor-output-coupled amp, I mainly focus on fixing tube amps. I have fixed about a dozen solid-state stereos, and I always check for DC offset immediately after the current/variac test is passed.
I understand the issues MarcelvdG addressed with the RC/scope test. But, the 2200u caps aren't tied to ground. Capacitor-output-coupled (SEPP) amps must still abide by the DC offset test, right? Expecting small mV range readings on the DMM? Funny, the Service Manual doesn't specify DC offset or how to bias the amp. There are 3 unknown trimpots (not the most important question in this amp at the moment).
Thanks!
This is my first capacitor-output-coupled amp, I mainly focus on fixing tube amps. I have fixed about a dozen solid-state stereos, and I always check for DC offset immediately after the current/variac test is passed.
I understand the issues MarcelvdG addressed with the RC/scope test. But, the 2200u caps aren't tied to ground. Capacitor-output-coupled (SEPP) amps must still abide by the DC offset test, right? Expecting small mV range readings on the DMM? Funny, the Service Manual doesn't specify DC offset or how to bias the amp. There are 3 unknown trimpots (not the most important question in this amp at the moment).
Thanks!
Again, most digital multimeters have a high input impedance usually at least 1M.
There is no DC offset adjustment on a capacitor coupled output amplifier. The offset you are measuring is inherent to the design, that is why the output capacitor is there in the first place.
If you really want check offset with your multimeter, you will need to measure the offset at the speaker terminals while a dummy load is connected.
There is no DC offset adjustment on a capacitor coupled output amplifier. The offset you are measuring is inherent to the design, that is why the output capacitor is there in the first place.
If you really want check offset with your multimeter, you will need to measure the offset at the speaker terminals while a dummy load is connected.
Ideally you should connect an 8 ohm power resistor to the output of the channel you want to measure, but when you simply short the headphone output, the resistors between loudspeaker outputs and headphone output will already get the output capacitors charged in a reasonable time.
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